Re: [net-next,v8,09/15] quic: add congestion control

On Wed, Jan 28, 2026 at 11:15 AM Simon Horman <[email protected]> wrote:
>
> This is an AI-generated review of your patch. The human sending this
> email has considered the AI review valid, or at least plausible.
>
> For local reproduction steps see https://netdev-ai.bots.linux.dev/ai-local.html
> ---
> quic: add congestion control
>
> This patch introduces quic_cong for RTT measurement and congestion
> control. It implements a congestion control state machine with slow
> start, congestion avoidance, and recovery phases using New Reno.
>
> > diff --git a/net/quic/cong.c b/net/quic/cong.c
> > new file mode 100644
> > index 000000000000..1a8b7f8db977
> > --- /dev/null
> > +++ b/net/quic/cong.c
>
> [ ... ]
>
> > +/* rfc9002#section-5: Estimating the Round-Trip Time */
> > +void quic_cong_rtt_update(struct quic_cong *cong, u64 time, u32 ack_delay)
> > +{
> > +     u32 adjusted_rtt, rttvar_sample;
>
> [ ... ]
>
> > +     /* rfc9002#section-5.3:
> > +      *   adjusted_rtt = latest_rtt
> > +      *   if (latest_rtt >= min_rtt + ack_delay):
> > +      *     adjusted_rtt = latest_rtt - ack_delay
> > +      *   smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
> > +      *   rttvar_sample = abs(smoothed_rtt - adjusted_rtt)
> > +      *   rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
> > +      */
> > +     adjusted_rtt = cong->latest_rtt;
> > +     if (cong->latest_rtt >= cong->min_rtt + ack_delay)
> > +             adjusted_rtt = cong->latest_rtt - ack_delay;
> > +
> > +     cong->smoothed_rtt = (cong->smoothed_rtt * 7 + adjusted_rtt) / 8;
> > +     if (cong->smoothed_rtt >= adjusted_rtt)
> > +             rttvar_sample = cong->smoothed_rtt - adjusted_rtt;
> > +     else
> > +             rttvar_sample = adjusted_rtt - cong->smoothed_rtt;
>
> The comment above references abs(smoothed_rtt - adjusted_rtt). Paolo Abeni
> suggested using abs_diff() from linux/math.h for this calculation in v3
> review, and it was acknowledged. Could abs_diff() be used here instead of
> the manual if-else?
>
> https://lore.kernel.org/netdev/[email protected]/
>
Wow, AI truly has a good memory.
Right, I somehow missed this. will apply abs_diff() here.

> [ ... ]
>
> > diff --git a/net/quic/cong.h b/net/quic/cong.h
> > new file mode 100644
> > index 000000000000..e6cfb0fa1b6c
> > --- /dev/null
> > +++ b/net/quic/cong.h
>
> [ ... ]
>
> > +struct quic_cong {
> > +     /* RTT tracking */
> > +     u32 max_ack_delay;      /* max_ack_delay from rfc9000#section-18.2 */
> > +     u32 smoothed_rtt;       /* Smoothed RTT */
> > +     u32 latest_rtt;         /* Latest RTT sample */
> > +     u32 min_rtt;            /* Lowest observed RTT */
> > +     u32 rttvar;             /* RTT variation */
> > +     u32 pto;                /* Probe timeout */
> > +
> > +     /* Timing & pacing */
> > +     u64 recovery_time;      /* Recovery period start timestamp */
> > +     u64 pacing_rate;        /* Packet sending speed Bytes/sec */
> > +     u64 pacing_time;        /* Next scheduled send timestamp (ns) */
> > +     u64 time;               /* Cachedached current timestamp */
>                                    ^^^^^^^^^^
>
> There appears to be a typo here - "Cachedached" should be "Cached".
Will fix this typo.

Thanks.